The present invention relates generally to current limiting protection circuits for the output of an amplifier, and more specifically to an improved controlled current limiter circuit.
A known and often used method of limiting current in a linear solid state circuit for the prevention of device failure either due to excessive current and/or excessive power is shown in FIG. 1. This circuit is generally used to protect the output of an operational amplifier from damage when the input is mishandled for example, by a short circuit to ground. The input to the input transistor Q1 is the output current of the operational amplifier. Under normal operations, the base to emitter voltage of the limiting transistor Q2 is less than that which would turn it on. When the output is grounded, Q2 turns on to limit the base voltage to Q1. The output current is limited to the base to emitter voltage of Q2 divided by the value of the resistance R1.
A typical example of a prior art device similar to that of FIG. 1 is shown in U.S. Pat. No. 3,967,207. The use of a diode and series resistor to provide current limiting once the breakdown voltage of the diode is achieved is illustrated in U.S. Pat. No. 3,737,800.
The prior art circuit FIG. 1 has three substantial shortcomings. The first is that the output current varies directly with the base-emitter voltage which is highly temperature dependant. Secondly the output current varies inversely with the value of the resistor R1. Since R1 is a small value for a high enough output current to make the circuit useful, it is implemented in monolithic circuits with a highly doped diffusion. This high doping gives a small value resistance and resonable size. The resistance increases substantially with temperature rises. The third disadvantage is that the output current during mishandling stays nearly constant when the source voltage V.sub.CC is increased. This means power dissipation is approximately proportional to the source voltage V.sub.CC during mishandling.
In an effort to make the current limiter more responsive to the difference between a source and output voltage, a voltage divider has been provided at the base of the limiting transistor Q2 as illustrated in FIG. 2. This includes resistors R2 and R3. A diode Z1 is provided at the base of the transistor Q2 to help determine the voltage at which the current limiting occurs. The limiting circuit of FIG. 2 is described specifically in U.S. Pat. No. 3,668,545. Although being an improvement over the prior art structure FIG. 1, the voltage divider R1 and R3 during normal use provides a current path between the source V.sub.CC and thus has power dissipation. Also the prior art circuit of FIGS. 1 and 2 do not provide a fixed minimum current when the difference between the source voltage V.sub.CC and the output voltage becomes extremely high.